It has long been recognized that amplification, no matter how non-linear and sophisticated, can improve speech comprehension in devices such as hearing aids only so much. Further progress depends on increasing the signal-to-noise ratio (SNR) of the hearing aid output. In general, the hearing-impaired need higher SNR than normal to understand speech. This need arises not only from hearing damage, but also as a necessary consequence of aging. For both men and women aged 70-79, fully 25% of this population may need 5.9 dB more SNR to hear normally. The requirements for 50% of the population aged 80-89 have been reported to be 7.0 dB for men and 5.6 dB for women. These numbers understate the problem by excluding hearing damage. Statistics in the literature are consistent with the hypothesis that the extra SNR requirement for the population at large is 50% greater than the numbers above. It should be borne in mind that these statistics apply to the entire population, not just the "hearing impaired".
The extra SNR requirements above are substantial. Each dB of SNR may improve the sentence intelligibility score by 15-20%. A hearing loss of 6-8 dB implies that people are not able to understand a speaker when a competing speaker is present at about the same distance from the listener. Any hearing loss at all implies loss of understanding when multiple competing speakers create an interference field equal in level to the targeted speaker. The inability to function normally in the common situations above can have a devastating effect on people's social life and outlook. The ill effects of this form of hearing loss are not confined to the aged. For example, anecdotal evidence abounds of the distress caused by the inability to hear multiple speakers at business meetings and lunches.
Many attempts have been made to increase signal to noise ratio for hearing aid users. Attempts to do so by frequency filtering have been unsuccessful, and in fact usually counterproductive because the frequencies to be filtered out are usually those of other speakers. These are precisely the frequencies needed to understand the speaker of interest. More recent attempts have focused on directional hearings aids with maximum sensitivity in the direction the user faces. Two types of aids have been considered: directional microphones and multiple microphones with beamforming.
Typical directional microphones produce 1-2 dB of intelligibility weighted gain for a diffuse noise field with exceptional microphones producing on the order of 4 dB. This is helpful, but not enough for the typical subject. Directional microphones can however synergistically provide inputs to beamformers. For example, if directional microphones attenuate an interferer by 2 dB, the problem confronting the beamformer has been reduced in severity by 2 dB.
Several types of beamformers have been used in an attempt to improve such hearing aids, but none have as yet produced satisfactory results. For example, fixed weight beamformers have been used. Such fixed weight beamformers sum weighted microphone outputs with weights constant in time. Fixed weight beamformers can provide only about 5 dB of intelligibility weighted gain in a diffuse noise field. Using fixed weights and five directional microphones, perhaps 7 dB is possible in a diffuse noise field.
Superdirective beamformers use large weights of different sign for summing microphone outputs. Use of superdirective beamformers can give perhaps 11 dB of intelligibility weighted gain with an eyeglass temple mounted array. There are two drawbacks to the technique: 1) it is sensitive to errors in construction, processing, and assumptions about the noise field; and 2) it can require focusing (taking into account the distance to the subject of interest) of the array to avoid distortion. Use of such superdirective beamformers requires careful tradeoffs between robustness and performance. These tradeoffs may significantly reduce the performance possible.
Adaptive beamformers have also been used. Such adaptive beamformers do not use weights constant in time. Rather, the weights change as required to maintain sensitivity in the desired direction and to minimize sensitivity in the direction of noise sources. Such adaptive beamformers can produce good results in anechoic environments with point interferers: 25-30 dB and more of intelligibility weighted gain are possible. Adaptive beamformers outperform fixed weight beamformers in the case of anisotropic diffuse noise because fixed weight beamformers are optimized for isotropic noise whereas adaptive beamformers optimize themselves for any anisotropic diffuse noise. Performance against isotropic diffuse noise is the same as that of fixed weight beamformers. Their major drawback is poor performance in reverberant environments. In such an environment, they can even degrade the signal and achieve a negative intelligibility weighted gain.